Within the field of semiconductor fabrication technology, the area of lithographic processing is characterized by a continuing need to decrease the minimum feature size of components within an integrated circuit. Therefore, each new generation of advanced integrated circuits has required an increase in lithographic resolution. Traditionally, optical lithography has been used to define integrated circuit patterns on semiconductor substrates. However, current optical lithography systems have great difficulty defining photoresist patterns with feature sizes of less than about 0.35 microns. A present need exists for the fabrication of integrated circuits having feature sizes approaching about 0.25 microns.
In view of the limitations of optical lithography systems, X-ray lithography technology is under development to meet the needs of the next generation of integrated circuits. The extremely short wave length of X-ray radiation eliminates the diffraction effects that limit the resolution of optical lithography systems. The elimination of diffraction effects, associated with the use of X-ray radiation, provides resolution capability far exceeding that of optical lithography. In addition, X-ray lithography also affords the advantage of a greater depth of focus and relative insensitivity to particulate contamination which may be present in the lithographic system during pattern generation. The ability of X-ray radiation to penetrate a wide variety of substances makes the X-ray lithography process less sensitive to many systematic defects that limit optical lithography, such as reflection, refraction, and particulate contamination.
At present, the preferred technique for X-ray lithography is proximity printing using a synchrotron radiation source. For proximity printing, it is necessary to make a mask having the same feature sizes as that desired to be printed on the semiconductor substrate. Thus, the mask making process is one of the key technologies necessary for the practice of X-ray lithography.
A major impediment in the development of X-ray masks has been the identification of materials, which meet the demanding criteria for high precision mask fabrication. For example, many materials will not absorb X-ray radiation to an extent sufficient for use as an X-ray absorbing layer. Typically, materials are selected which have a high atomic number and are capable of being patterned by high resolution methods, such as E-beam lithography and reactive ion etching. Initially, gold was used as the X-ray absorbing material and deposited onto a membrane layer. Gold possesses many desirable physical characteristics, such as smoothness and low internal stress. However, gold is a contaminant in a semiconductor device, and therefore gold and gold alloys are being abandoned in favor of refractory metals.
Present X-ray mask development work has centered on refractory metals, such as tungsten and tantalum, for use as an X-ray absorbing layer. In addition, several materials have been developed for use as membrane layers because of their relative durability and transparency to X-ray radiation. For example, membranes have been fabricated from silicon, silicon nitride, silicon carbide, and diamond.
While the foregoing materials possess the necessary absorption and transmission characteristics, a major drawback to their use has been the creation of high internal stress in an X-ray absorbing mask fabricated from these materials. High internal stress in an X-ray absorbing mask creates spatial distortions which compromise the pattern definition capability of X-ray lithography. Accordingly, further development in mask fabrication techniques is necessary to minimize the degree of internal stress present in advanced X-ray absorbing masks.